The present invention relates to magnetoresistance sensors and particularly to sensors exhibiting extraordinary magnetoresistance greater than 100% at 500 Gauss at room temperature in inhomogeneous narrow-gap semiconductors.
Various types of sensors exhibiting magnetoresistive characteristics are known and implemented in systems, particularly for reading of information signals recorded in magnetic media such as tapes, drums and diskettes. Sensors are also used to monitor shaft position and/or rotation and for proximity switches.
These sensors typically comprise a block made of a ferromagnetic alloy exhibiting high magnetoresistance. A recording medium, for example, passing in close proximity to such a sensor causes variations in the magnetic field at the point of the read head and hence variations of the electrical resistance of the magnetoresistive sensors.
It has recently been described in U.S. Pat. No. 5,965,283 entitled xe2x80x9cGMR Enhancement in Inhomogeneous Semiconductors for use in Magnetoresistance Sensorsxe2x80x9d that embedding a conducting material in a matrix of high carrier mobility semiconductor material will increase the magnetoresistance of the combined semiconductor material with embedded conducting material.
Magnetoresistive sensors are critical components in several important technologies including high-density information storage as described, for example, in an article by J. A. Brug et al., entitled xe2x80x9cMagnetic recording head materials,xe2x80x9d in MRS Bulletin, Vol. 21, pages 23-27, 1996. Another important technology is position/speed monitoring in mechanical devices, such as described in an article by J. P. Heremans, entitled xe2x80x9cMagnetic Field Sensors for Magnetic Position Sensing in Automotive Applications,xe2x80x9d in Mat. Res. Soc. Symp. Proc., Vol. 475, pages 63-74, 1997 and in an article by N. Kuze et al. in III-V Review, vol. 10, 28-31 (1997). The technological impact of such sensors is currently constrained by the magnitude of their room temperature (300 K) magnetoresistance (MR). Efforts to improve their room temperature response are focused on two classes of magnetic materials, artificially layered metals, like those described by P. M. Levy in an article entitled xe2x80x9cGiant Magnetoresistance in Magnetic Layered and Granular Materialsxe2x80x9d in Solid States Physics, vol. 47, pages 367-462 (1994), which exhibit Giant MR (GMR) (see, W. F. Egelhoff et al., xe2x80x9cMagnetoresistance values exceeding 21% in symmetric spin valves,xe2x80x9d Journal of Applied Physics, vol. 78, pages 273-277 (1995)) and the manganite perovskites as described by C. N. R. Rao et al., in a book entitled xe2x80x9cColossal Magnetoresistance, Charge Ordering and Related Properties of Manganese Oxidesxe2x80x9d World Scientific, Singapore (1998), which show Colossal MR (CMR) (see, also, S. Jin et al., xe2x80x9cColossal magnetoresistance in La-Ca-Mn-O ferromagnetic thin films,xe2x80x9d Journal of Applied Physics, Vol 76, pages 6929-6933 (1994)). CMR has also been reported for non-magnetic silver chalcogenide semiconductors in an article by R. Xu et al., entitled xe2x80x9cLarge magnetoresistance in non-magnetic silver chalcogenidesxe2x80x9d, Nature, vol. 390, pages 57-60 (1997).
In accordance with the teachings of the present invention, a composite of non-magnetic InSb, a high mobility, narrow-gap semiconductor and metal, exhibits room temperature MR orders of magnitude larger than that obtained to date with other materials. Although InSb exhibits moderate MR in the unpatterned state, embedded metallic inhomogeneities engender room temperature MR""s as high as 100%, 9,000% and 750,000% at fields of 0.05, 0.25 and 4.0T, respectively. This Extraordinary MR (EMR) occurs because at H=0 the conducting inhomogeneity is a short circuit, as expected, but at high field it acts, counter-intuitively, as an open circuit. See, Solin et al., Science, vol. 289, 1530 (2000) and Thio et al., xe2x80x9cGiant Magnetoresistance Enhancement in Inhomogeneous Semiconductors,xe2x80x9d Applied Physics Letters, Vol. 72, pages 3497-3499 (1998).
In contrast to the negative MR observed in layered metals or manganite perovskites, the MR on a non-magnetic semiconductor is positive, see T. Thio, et al., xe2x80x9cGiant magnetoresistance in zero-bandgap Hg1xe2x88x92xCdxTe,xe2x80x9d Physical Review B, vol. 57, no. 19, pages 12239-12244 (1998), and comprises a physical and a geometric contribution. See H. H. Wieder, Hall Generators and Magnetoresistors, Pion Ltd., London (1971) and R. S. Popovic, Hall effect devices, Adam Hilger, Bristol (1991). The physical MR results from the orbital motion of the charge carriers caused by the Lorentz force. The physical MR is determined by material parameters such as the carrier mobility and density (and their magnetic field dependences). The geometric MR depends on the shape of the sample and the placement of electrical contacts and increases quadratically with mobility and field at low field. Thus high mobility narrow-gap semiconductors, for example, as described in W. Zawadzki, xe2x80x9cElectron transport phenomena in small-gap semiconductors,xe2x80x9d Adv. Phys., vol. 23, pages 435-522 (1974), such as InSb and Hg1xe2x88x92xCdxTe (where x is preferably approximately 0.1), are attractive proving grounds for geometric effects. It is also well known that inhomogeneities can enhance the MR of a material, as described by C. Herring, xe2x80x9cEffect of random inhomogeneities on electrical and galvanomagnetic measurements,xe2x80x9d Journal of Applied Physics, vol. 31, pages 1939-1953 (1960) and A. Y. Shik, xe2x80x9cElectronic Properties of Inhomogeneous Semiconductorsxe2x80x9d Gordon and Breach, Amsterdam (1995), but the mechanism and/or the geometry differed from those of the present invention and yielded room temperature MR values orders of magnitude lower than the values achieved when practicing the present invention.
In accordance with the teachings of the present invention, the room temperature magnetoresistance of a semiconductor can be enhanced greatly in a van der Pauw disk geometry with an embedded concentric metallic inhomogeneity. Similar enhancement is achieved when embedding inhomogeneities in semiconductors having shapes other than that of a disk, such as a bar geometry or thin film. Also, the embedded inhomogeneity need not be concentrically located within the disk (centered), but may be off-center. Moreover, either or both of the van der Pauw plate and embedded inhomogeneity may be other than disk shaped. The van der Pauw plate is a closed surface. The inhomogeneity can be of an arbitrary shape.
Adjusting the ratio of the radius of an embedded cylindrical inhomogeneity to the radius of the van der Pauw disk varies the magnetoresistance of the disk as a function of the ratio. Optimizing the ratio results in the disk sensor exhibiting extraordinary magnetoresistance at room temperature. Similarly, optimizing the ratio of the area of an embedded inhomogeneity to the area of the van der Pauw plate in the case of a non-cylindrical inhomogeneity and a non-disk-shaped plate results in a sensor exhibiting extraordinary magnetoresistance at room temperature.
The present design exhibits very high thermal stability. The design also provides the potential to be manufactured at a much lower unit cost than conventional sensors and the capability of operating at speeds of up to 1,000 times higher than sensors fabricated from magnetic materials.
The higher operating speed achievable using the present design will enable industry to build a magnetic disk drive that is capable of storing a Terabit or 1,000 Gigabits of data per square inch.
Another aspect of the present invention takes into consideration that fabricating cylindrical apertures in a van der Pauw disk and fabricating cylindrical inhomogeneities, particularly at mesoscopic size scale, are difficult tasks. Accordingly, a technique is described which maps the cylindrical plate of the van der Pauw disk with an internal cylindrical inhomogeneity into an electrically equivalent rectangular plate with an external rectangular inhomogeneity or shunt thereby enabling fabrication of simpler rectangular or polygon shaped components in order to make an extraordinary magnetoresistance sensor. The mapping technique is applicable for converting any generally sensor design into a polygonal shaped sensor design.
A principal object of the present invention is the provision of a magnetoresistance sensor comprising a semiconductor material containing a conductive inhomogeneity where the dimensions of the inhomogeneity are selected to optimize the magnetoresistance of the sensor.
Another object of the present invention is the provision of a mapping technique to map the shape of the semiconductor material and embedded conducting inhomogeneity of an EMR sensor into a polygon, and preferably a rectangular shape with an external conducting inhomogeneity or shunt, dimensioned to facilitate fabrication of the EMR sensor.